1. Field of the Invention
The invention relates to an apparatus for producing hollow bodies of thermoplastic. The invention also relates to a method for producing hollow bodies of a plastic in a hollow body blowing process.
2. Background Information
The containers of tinned sheet-iron or mixed sheet metal, glass, or ceramic that were typical in the past are increasingly being replaced with plastic containers. Especially for packaging fluid substances, such as beverages, oil, cleaning utensils, cosmetics, and so forth, plastic containers are used primarily. The low weight and lower costs surely play a not inconsiderable role in this substitution. The use of recyclable plastic materials and the overall more-favorable total expenditure of energy for producing them also contribute to increasing consumer acceptance of plastic containers, especially plastic bottles.
The production of plastic containers, especially plastic bottles, is done by extrusion blowing, in particular hollow body blowing, for instance tubular film blowing. In this process, a preform such as a tube, extruded in a single layer or multiple layers, is placed in blow molds, inflated by overpressure via a blow mandrel, and hardened by cooling. The automatic blow molder used for this purpose as a rule has a plurality of parallel-connected blow molds, which make it possible to produce a plurality of plastic containers simultaneously in one operation. Each blow mold is assigned a separate blow mandrel, which can be put into position in a way suitable for the process.
In many cases, hollow plastic bodies, such as bottles or canisters, but also industrial parts, have one or more openings, which are for a particular purpose in later use. Often such openings are embodied such that they are closable by suitable closing devices. These can for instance be screw closures or snap closures. An essential demand the closing technology must meet is tightness to liquids, gases and solids in powdered or granular form. In plastic packages, the tightness is typically achieved by creating smooth, dimensionally accurate sealing faces adapted to one another in both the hollow body and the closure. It is known that the sealing quality can be improved still further by the choice of the correct combination of materials for the hollow body and the closure.
Producing openings designed in this way typically is done simultaneously with the process of producing the hollow body. In it, the opening for introducing the inflation medium that generates the overpressure, which in most cases is compressed air, is designed at the same time such that it also meets the later demands made in terms of the use of the hollow body. In these cases, the sealing faces, which can be present both on the end face and on the preferably cylindrical or conical inner wall of a neck extending around the edge of the opening, must be produced. This is done in a so-called calibration operation, that is, by means of a dimensional adaptation of a portion of the blow mandrel, acting as a calibrating device, with the orifice region of the blow molds and with the predetermined wall thickness of the preform placed in the blow molds. The blowing and calibration mandrel furthermore has the task of pinching off process-created protruding parts of the preform in the upper edge region of the orifice, except for a minimal residual thickness of the hollow body, so that they can be pinched off perfectly in a deburring process following the inflation operation.
For both operations, that is, calibrating the opening and pinching off protruding parts, it is necessary for the blowing and calibration mandrel to assume a very accurate, replicable position in three planes relative to the orifice of the blow mold. In particular, the axis of the blowing and calibration mandrel must match the axis of the orifice of the cavity in the blow mold; the cutting ring face of the blowing and calibration mandrel must be parallel to the neck blade face of the blow mold, and the terminal height position of the blowing and calibration mandrel relative to the orifice of the blow mold must always assume exactly the position in which not only the dimensional tolerances of the finished hollow body are adhered to but also the pinching off of the protruding parts is assured. In practice, this is achieved by adjusting a mechanical end stop for the blowing and calibration mandrel. The adjustment is done manually, in accordance with a visual assessment of the pinching-off operation and dimensional checking of the orifice of the hollow body by the operator or operators. In a single blowing device that has only a single blowing and calibration mandrel, both demands in terms of quality can still be met relatively simply and simultaneously with regard to the opening and to the protruding parts. In automatic blow molders with a plurality of parallel blow molds and blowing and calibration mandrels associated with them, however, the adjusting operation can be accomplished only with great difficulty and is very time-consuming. In view of the demands for quality made of the pinching-off operation, the individual blowing and calibration mandrels must all be adjusted to the correct height relative to the orifices of the cavities in the blow molds. In this respect it must be noted that dimensional differences at the orifices of the individual blow molds, already dictated by the usual production tolerances or by wear, make different height adjustments of the blowing and calibration mandrels unavoidable. Because of the risk of injury to the operator, the mechanically cooperating components of these multiple automatic blow molders are protected against unauthorized or unintentional access by covering devices. However, these protective structural provisions are an obstacle to practicable adjustment, which should advantageously be done with the machine running.